1. Field of the Invention
The present invention relates to a microscope or a biological microscope having an auto-focus function for an observational optical system using a focus error signal.
2. Description of the Related Art
An auto-focus device has been known, which executes automatic focus control by driving an objective lens of a microscope by an actuator. This device detects a focus error from a focal point of the objective lens of the microscope, moves the objective lens by the actuator such as a piezoelectric element according to a detection signal of the focal shift, and performs focusing (refer to JP-A-5-88072, for example).
Such the auto-focus device enables auto-focus on a surface of an object to be observed in a case where surface observation is performed by a metallurgical microscope or the like. However, a microscope such as a biological microscope has specialty of observing the object to be observed through a cover glass. In the related art auto-focus device, although the auto-focus onto the surface of the cover glass is possible, the auto-focus onto the object to be observed, such as a cell, which is placed inside the cover glass is impossible. Further, in a case where an oil immersion lens is used as an objective lens, light reflection does not occur at a boundary between the cover glass and oil, so that even the focus onto the surface of the cover glass becomes impossible.
Furthermore, in such the auto-focus device of the related art, a detection optical system for obtaining the detection signal includes an objective lens. Therefore, in a microscope using a plurality of objective lenses, when the objective lens in use is switched, its numerical aperture changes, so that an appropriate detection signal cannot be obtained. Further, such the microscope cannot deal with the shift of an optical axis due to switching of the objective lens. Further, a biological microscope has specialty of observing an observation object through a cover glass, so that a focus error detecting optical system having high accuracy is required in order to perform auto-focus onto the observation object such as a cell placed inside the cover glass. Therefore, there are circumstances that an appropriate focus error detecting characteristic must be always maintained regardless of the objective lens to be selected.